Snow cover data updated

The MODIS-based snow cover data set has been updated to version 6. This new version includes several improvements (see here for more details). In the table env_data_ts.snow_modis, data are all those of V.6. The values stored in the column snow_modis of the table main.gps_data_animals have been updated accordingly. Version V.5 is still stored on the EURODEER server for comparison studies, if needed.